A patients with marked chronic hypokalemia (potassium, 1.7 to 2;3 meg/litre) and sodium depletion secondary to lazative abuse and dietary inadequacy was studied with respect to the renin-aldosterone system during sequential potassium and potassium-plus-sodium replacement. Extreme hyperreninemia of 20 Goldblatt units X 10-minus 4 was reduced to 0.9 with potassium replacement alone. Aldosteron excretion (15.8 mug/24 h) was initially low for a sodium-deprived state and high for a potassium-deprived state; it increased with potassium administration, but this rise was opposed by decreases in renin secretion induced by potassium and sodium administration. The results provide clinical confirmation of a dual effect of potassium on aldosterone secretion, with renin as a mediator.